The junction between a metal and an N-type semiconductor may be either rectifying or ohmic--i.e., non-rectifying--depending on the characteristics of the materials in the metal-semiconductor system. As pointed out by R. Muller et al in Device Electronics for Integrated Circuits (John Wiley and Sons: New York), 1977, the work function distinguishes a rectifying junction from an ohmic junction. The work function is a measure of the average energy needed to raise an electron from the Fermi energy level to an energy level at which the electron is just free of the influence of the system. If the work function of the metal is greater than that of the semiconductor when the materials are not interacting, a rectifying junction is formed when they interact.
For the rectifying junction, a charge depletion region consisting of bound positive charges exists on the semiconductor side of the interface. In an idealized system, the electron associated with the bound positive charges are situated in the metal right next to the interface--i.e., no more than several monolayers into the metal. This sets up an induced electric field directed across the depletion region toward the interface where the field essentially ends. As a result, a built-in voltage .phi..sub.I whose magnitude depends on the N-type dopant concentration in the semiconductor exists across the depletion region. The built-in voltage represents an energy barrier q.phi..sub.I that electrons in the semiconductor conduction band must overcome to cross the interface and enter the metal. q is the electronic charge. This energy barrier can be reduced by applying an external voltage that is positive with respect to the semiconductor. As the applied voltage is increased, a point is eventually reached at which electrons flow freely from the semiconductor to the metal. .phi..sub.I is thus indicative of the conductive forward voltage drop .phi..sub.SH across the rectifying junction. However, .phi..sub.I is often difficult to measure directly.
A parameter more readily ascertainable from the current/voltage characteristics of the system is the Schottky barrier height, represented as .phi..sub.B in electrostatic potential units or as q.phi..sub.B in energy units, that electrons in the metal at the Fermi level must overcome to cross the interface and enter the semiconductor. Because .phi..sub.B is greater than .phi..sub.I and is not affected to any significant degree by the applied voltage in the idealized system, very few electrons in the metal can enter the semiconductor. .phi..sub.B is also linearly related to .phi..sub.I and therefore provides a direct measure of .phi..sub.SH.
Numerous types of metal-semiconductor rectifying elements, generally referred to as Schottky diodes, have been considered. The oldest Schottky diodes utilize aluminum as the metal and doped silicon as the N-type. semiconductor. These basic Al-Si diodes are manufactured relatively easily. However, they degrade relatively fast as the aluminum intermixes with the silicon.
In view of this problem, other materials have been sandwiched between the aluminum and silicon in more recent Schottky diodes. In one of the principal arrangements, a diode is formed on a semiconductor body which has an appropriately doped N-type semiconductive region. A layer of a metal silicide adjoins the N-type region to define a rectifying junction at the interface between the N-type region and the silicide layer. The junction is still referred to as a metal-semiconductor or Schottky junction since metal silicides have been conventionally thought to be metallic in nature. The silicide is typically platinum silicide whose q.phi..sub.B with N-type silicon is 0.79-0.83 electron volt (eV) depending on the dopant concentration. As, for example, described by W. Rosvold in U.S. Pat. No. 3,855,612, nickel is often included with platinum in the silicide layer. The nickel reduces the junction q.phi..sub.B since the nickel q.phi..sub.B is about 0.63-0.67 eV with N-type silicon.
The thickness of the silicide layer is typically on the order of 1,000 angstroms and is normally no less than about 500 angstroms. This thickness is employed because of the conventional belief that this much silicide is needed to make the silicide layer stable and to avoid continuity defects such as pinholes in it.
A conductive non-silicide layer typically consisting of a barrier metal such as tungsten or titanium-tungsten ohmically adjoins the silicide layer across from the rectifying junction. The barrier metal substantially prevents the materials on either side of it from diffusing through it to the other side. Also, the barrier metal does not diffuse into the silicide. Finally, an aluminum layer normally overlies the barrier metal.
In one method for manufacturing a rectifying element of the above type, a metallic layer composed of at least two selected metals such as platinum and nickel is deposited on an exposed surface of an N-type silicon semiconductive region in a semiconductor body. The resultant structure is heated to a suitable temperature to cause the metal to react with adjacent silicon and form the metal silicide layer. Any deposited metal that has not reacted to form silicide is suitably removed. The non-silicide layer is then formed after which the aluminum layer is created.
While known Pt-based silicide Schottky diodes perform well without significant junction degradation, they are relatively expensive because of the high cost of platinum. In a Schottky diode whose silicide layer is constituted with two or more metals, controlling the ratio of these metals to one another in the deposition target employed in forming the metallic layer later converted to silicide is also difficult. It is desirable to have a less expensive silicide Schottky-type diode whose .phi..sub.B can be varied so as to appropriately tailor .phi..sub.SH more simply than in the prior art.